1. Technical Field
This invention relates to ultrafiltration membranes and the methods for making and using such ultrafiltration membranes to provide gaseous mixtures separation media at elevated temperatures. In one aspect, the invention relates to such ultrafiltration membranes to provide gaseous mixtures separation media having superior hydrogen selectivity.
2. Background
Porous ceramic membranes composed of metallic oxides, such as oxides of aluminum, silicon, zirconium, and titanium, have been suggested and used for high temperature separations processes ranging from &lt;200.degree. C. to &gt;600.degree. C. The membrane could be in the form of tubular, disk, or monolithic configurations. The effective membrane layer is usually very thin, e.g., such as typically 1 to 100 microns and is supported by a very porous, thick ceramic substrate. The porous structure of the membrane layer is formed by the metal oxide network, wherein a molecule can selectively permeate through to achieve the high temperature separations processes.
Existing commercial ultrafiltration membranes are typically prepared by depositing a suspended sol containing aluminum or silicon onto a porous ceramic substrate with a pore size of .gtoreq.20.2 micron as support. After deposition, the material is calcined at a temperature higher than about 450.degree. C. to obtain a pore size ranging from 40 .ANG. to &lt;0.2 micron. Details about ceramic membrane synthesis and applications can be found in Chapter 2 of Inorganic Membrane Synthesis, Characteristics and Applications (Van Nostrand Reinhold, New York, 1991) by R. R. Bhave.
Membranes also can be prepared by other methods. For example, vycor glass membranes are prepared from etching of the glass. The pore size in a vycor glass membrane depends on the control of the defect size resulting from the etching.
To date, 40 .ANG. is the smallest nominal pore size commercially available for inorganic membranes. By nominal pore size is meant the flow-weighted pore size as determined by the method described by D. E. Fain in "A Dynamic Flow-Weighted Pore Size Distribution," Proceedings of the First International Conference on Inorganic Membranes, Montpellier, France, 1989.
A membrane with a nominal pore size as small as 40 .ANG. can perform gas separations based upon Knudsen diffusion, i.e., the selectivity approximating the square root of the molecular weight ratio. For example, in the separation of a gaseous mixture containing hydrogen and nitrogen, the selectivity of the membrane would be about 3.74 (square root of 28/2). In a simplified explanation of the selectivity, one nitrogen molecule leaks through with every 3.74 molecules of hydrogen permeated through the membrane. A separation efficiency as low as 3.74 is insufficient for most industrial gas separation processes. Details on the gas separations using microporous ceramic membranes can be found in "High Temperature Separation of Binary Gas Mixtures Using Microporous Ceramic Membranes," by J. C. S. Wu, D. F. Flowers, and P. K. T. Liu, J. Mem. Sci. Vol. 77, pp. 85-98 (1993).
In addition to selectivity, membrane stability at the elevated temperature (i.e., 200.degree. C. and above) is another critical factor. To achieve membrane material stability, the oxide normally is required to be thermally treated at a temperature higher than the contemplated process application temperature because particles of the oxide network tend to sinter (aggregate) under long term exposure to a temperature higher than the preparation temperature. This sintering phenomenon is purely thermodynamic since particles tend to aggregate together to minimize their free energy. Treatment of the material at the contemplated or proposed process application temperature makes the material reach a "steady state" corresponding to the operation temperature. A similar material stability issue has been discussed in the literature for these materials in non-membrane configurations, such as for activated alumina (Oxides and Hydroxides of Aluminum, Alcoa Technical Paper No. 19, Revised, by K. Wefers and C. Misra, Alcoa Laboratories, Aluminum Company of America, Pittsburgh, Pa., 1987) and for other porous oxides.
The membrane material aggregation process of sintering involves a serious drawback for separations in that the densification of the porous membrane usually degrades permeate flux. Eventually, the membrane material becomes non-porous and is usually associated with cracks which reduce separation selectivity dramatically.
In the past few years, several techniques have been proposed to modify metal oxide membranes via pore size reduction of the existing membrane by methods other than sintering to enhance separation selectivity at the high temperature. Pore constriction processes by chemical vapor deposition of metal oxides or carbon, by precipitation of metals, or by in-situ crystallization of zeolite have been proposed previously. In addition to the pore size reduction via pore constriction, these techniques also have been applied to introduce an additional layer, of a smaller pore size, onto the existing membrane layer. Improvement of the separation selectivity through these modifications has been reported in the literature.
For example, using chemical vapor deposition to reduce pore size, Okubo and Inoue ("Introduction of Specific Gas Selectivity to Porous Glass Membranes by Treatment With Tetraethoxysilane," in J. Membrane Science Vol. 42, p.109 (1989)) deposited a silicon-containing layer onto a porous glass membrane at 200.degree. C. using tetraethyl orthosilicate (TEOS) as a precursor. The membrane thus developed exhibited hydrogen permeance of 0.0447 m.sup.3 /m.sup.2 /bar/hr and selectivity of .about.11 over nitrogen at the pursued gas separation temperature of 200.degree. C. Okubo and Inoue further suggested that the separation was based upon activated diffusion through the microporous membrane and Knudsen diffusion through cracks and/or macropores. No deposition or separation studies at temperatures higher than 200.degree. C. were reported.
Theoretically, one could form a dense film as a membrane; thus, migration of particles could be minimized. The dense film suffers a low permeate flux, although the selectivity could be improved dramatically. The extreme case of the dense structure involves a dense film where molecules pass through the membrane based on the solid-phase solution-diffusion mechanism. Under these circumstances, the selectivity could be extremely high while permeability is too low to become practical. For example, in the work reported by Gayalas et al., ("Deposition of H.sub.2 -Permselective SiO.sub.2 Films," by G. R. Gayalas, C. E. Megiris, and S. W. Nam in Chemical Engineering Science Vol. 44(9), p.1829 (1989)), a dense phase membrane applied through vapor phase deposition at 450.degree. C. was suggested with a selectivity number of several thousands for hydrogen over nitrogen, while the permeance of hydrogen ranged from 0.11 to 0.33 m.sup.3 /m.sup.2 /atm/hr at 450.degree. C. A tremendously high surface area of this dense film type membrane was required for operation in petrochemical and oil refinery operations handling a large volume of streams.
Modified membranes with SiO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3, and B.sub.2 O.sub.3 were produced by Tsapatsis et al. ("Synthesis of Hydrogen Permselectivity SiO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3, and B.sub.2 O.sub.3 Membranes from the Chloride Precursors," in Ind. Eng. Chem. Res. Vol. 30(9), p. 2152 (1991). The metallic precursors included chlorides of the metals. The deposition temperatures varied, e.g., 600.degree.-800.degree. C. for SiCl.sub.4, 450.degree. -600.degree. C. for TiCl.sub.4, 450.degree.-800.degree. C. for AlCl.sub.3, and 100.degree.-450.degree. C. for BCl.sub.4. The carrier gas included water to convert the metallic chloride into metal oxide and hydrogen chloride during deposition. Again, the hydrogen permeance ranged from 0,002 to 0.21 m.sup.4 /m.sup.2 /atm/bar. Further treatment at a higher temperature, called thermal annealing, resulted in a lower permeate flux because of the densification of the membrane structure. For example, in Gayalas et al., U.S. Pat. No. 4,902,307, the hydrogen flux decreased from 11.0 m.sup.3 /m.sup.2 /atm/hr (after deposition at 450.degree. C.) to 4.2 m.sup.3 /m.sup.2 /atm/hr (after the film was heated for 18 hours at 600.degree. C.), and then to 2.3 m.sup.3 /m.sup.2 /atm/hr (after one additional day at 600.degree. C.), while the nitrogen flux increased to 0.28 m.sup.3 /m.sup.2 /atm/hr. The permeance data presented in FIG. 3 of the Gayalas et al. patent, however, was on the order of 10.sup.-2 to 10.sup.-3 m.sup.3 /m.sup.2 /atm/hr for hydrogen at 200.degree.-600.degree. C.
In addition to metal chlorides, SiH.sub.4 has been used by Gayalas et al. (Gavalas, Megiris, and Nam, 1989) as precursors with the carrier gas containing oxygen. SiH.sub.4 was converted to SiO.sub.2 during the deposition at 450.degree. C. (T. Ioannides & G. R. Gavalas, "Catalytic Isobutane Dehydrogenation in a Dense Silica Membane Reactor," J. Mem. Sci. Vol. 77, page 207 (1993)) and (G. E. Megeris and J. H. E. Glezer, "Synthesis of H.sub.2 -Selective Membrane by Modified Chemical Vapor Deposition, Microstructure and Permselectivity of SiO.sub.2 /C/Vycor Membranes," Ind. Eng. Chem. Res. Vol. 31, pp. 1293-1299 (1992)).